In the past, pilferage detection systems have been provided in which a magnetic or electromagnetic (resonant circuit) marker is placed in or on an article to be protected. If the marker is not removed or deactivated, the marker is detected by an electronic system located at the exit from the protected area.
While a great variety of antipilferage systems exist for the utilization of a magnetic or resonant circuit marker, all of the present systems have troublesome false alarms due to the high level of magnetic and electromagnetic noise in the environment. Such alarms are due to transient electromagnetic noise generated, for instance, by fluorescent lamps, the turning on and off of electrical machinery and, in general, by any unshielded electromagnetic radiation which reaches the detector portion of the system. As a result of such unwanted noise, George Lichtblau and others developed a number of filtering and signal discrimination systems to eliminate noise due to electromagnetic transients in the protected area. Representative Lichtblau systems are described in U.S. Pat. Nos. 3,810,147; 3,828,337; 3,863,244; 3,913,219; 3,938,044; 3,961,322; 3,967,161; 4,021,705; 4,117,466; 4,168,496; 4,243,980; 4,251,808; 4,260,990 and 4,498,076.
The problems associated with the utilization of a magnetic marker are, in many respects, similar to those problems associated with the utilization of a resonant circuit (radio frequency) marker. FIG. 1 1A illustrates the typical antipilferage system which uses a magnetic marker and FIG. 1B illustrates a similar system using a resonant circuit marker. In both cases, one antenna system produces an electromagnetic time varying field in the area of interest and a second antenna system monitors this field for disturbances caused by the specific type of "marker".
In both types of systems, the disturbance caused by a real marker is much shorter in time than the modulation period of the applied electromagnetic field. In the magnetic case, the field is amplitude modulated and in the resonant circuit system the radio frequency field is frequency modulated. In both cases, the basic modulation rate (and radio frequency carrier) must be removed from the monitor signal prior to further signal processing and signal discrimination.
The resonant circuit (swept radio frequency) systems are subject to problems due to spurious resonances in nearby electrical and mechanical equipment, beat-notes due to local radio transmitters and noise from fluorescent lamps, high frequency noise from nearby computers and point-of-sale case registers, and high frequency impulse noise from arcing electric motors. On the other hand, there are very few objects in retail stores which closely resemble a high "Q" resonant circuit. Magnetic based antipilferage systems are subject to many of the same type of noise sources as the resonant circuit systems. Such noise sources as arcing electric motors, point-of-sale cash registers, laser scanners, and computer printers generate large amounts of low frequency electrical noise which fall in the same frequency range as the signals produced by the magnetic markers. In addition, the magnetic based systems are constantly subject to disturbances caused by nearby magnetic materials which are present in retail stores. Supermarket checkout counters and shopping carts are almost always made of steel and other magnetic materials. The unwanted signals are not only due to the presence of magnetic materials other than the markers in the protected area, but also due to vibration of nearby magnetic materials caused, for instance, by rolling supermarket carts. The physical vibrations are then translated in spurious signals within the detection portion of the system. There have been many systems developed and patented to detect the signal produced by a resonant circuit within a swept radio frequency field. The Lichtblau earlier patents illustrate many of the problems and solutions. Likewise there have been many systems developed and patented to detect the signal produced by a magnetic marker. A very early technique was suggested in French Pat. No. 763,681 issued to P. A. Picard in 1934. Picard detected the marker by first filtering out many of the lower order harmonics of the field generating signal and then detecting a single harmonic i.e., the 13th harmonic of 50 Hertz. Picard also suggested comparing the amplitude ratio of several harmonics rather than just the amplitude of a single harmonic. Lastly he suggested that the phase of the signal produced by the magnetic marker be compared with the phase of the field producing means to aid in further discrimination.
In U.S. Pat. No. 3,631,442 issued to R. E. Fearon on Dec. 21, 1971, and U.S. Pat. No. 3,747,086 issued to G. Peterson on July 17, 1973, the magnetic field was generated using two separate frequencies and the marker served as a nonlinear mixer which gave rise to a third frequency which was the sum and difference between the two applied frequencies. The signal received by the magnetic field sensor was passed through a very narrowband filter which passed the difference (or sum) frequency.
Similar to Picard in U.S. Pat. No. 3,820,103, Fearon suggested detcting harmonics of the applied signal; however, the harmonics were detected in a synchronous detector phase-locked to the field generator so that any external noise not synchronized in frequency and phase with the magnetic field generator would be averaged out to zero.
In U.S. Pat. No. 3,665,449 issued to T. J. Elder, et al on May 23, 1972, it was recognized that the signal generated by a magnetic marker normally occurs slightly after the applied magnetic field passes through zero. Therefore, the detection system was turned on (gated) only during the portion of the modulation period when the magnetic field was near zero. In addition, the shape of the marker signal as a function of time was examined as compared to the harmonics representing the same signal.
In U.S. Pat. No. 3,983,552 issued to P. Bakeman and A. Armstrong on Sept. 28, 1976, a special magnetic marker was required and the detection system looked for only even harmonics of the applied field.
In all of the above systems, only a small part of the actual signal produced by the marker was used for signal discrimination. In addition, none of the previous systems for either resonant circuit or magnetic marker systems addressed techniques for removing high level impulse noise from the monitor systems or for automatically maintaining a minimum signal to noise ratio prior to signal discrimination.
It is quite clear that the limiting factor in sensitivity to detect a resonant circuit or magnetic marker is the relationship between the signal and the noise. If there were no noise, signal discrimination would be extremely simple. All of the previous systems looked only for the signal produced by the marker and did not examine the nature of the noise. All of the filters described in the previous patents were linear and therefore subject to high level ringing if driven by impulse type noise (automobile ignitions, etc). In addition, none of the previously described patents suggested any way to automatically adjust the sensitivity of the detection system to compensate for external noise picked up by the system.